It is well known that lightweight yet strong and durable structures ranging from simple to complex shapes can be formed of composite materials comprising fibers in a matrix of cured resin. (Concrete and wood, for example, are also considered composite materials, but are outside the scope of this invention.) Perhaps the most common composite material of this type is that referred to as “fiberglass,” or “glass-reinforced plastic,” which typically comprises glass fibers in a matrix of a curable resin, e.g., polyester, vinylester or epoxy resin.
Typically, the resins forming the matrix are in fact mixtures of resin per se with a “hardener” or “cross-linker”, that is, a chemical which when mixed with the resin (and possibly heated, as discussed below) causes a chemical reaction to take place whereby the molecules of the resin per se are cross-linked to one another and to the fibers in a “curing” step, such that a solid is formed.
In one common process for fabricating fiberglass structures, any or several of a wide variety of fabrics, both woven and non-woven, made up of fibers of glass, are disposed in a mold and impregnated with a hardening resin, typically polyester, vinylester or epoxy resin, and the resin allowed or caused to cure. Other materials are of course known for both the fabric and the resin; common choices for the fabric are aramids, such as that sold as Kevlar, and carbon fiber materials. It is also known to provide the fibrous materials in unidirectional form, that is, not as fabric per se, but commonly as tapes comprising a large number of individual filaments, or as yarns.
Various techniques are also known for application of the resin to the fibrous material. In one common process, dry layers of fabric or unidirectional fibers can be coated with liquid resin after disposition in the mold. The resin can be applied by brush or roller to the fabric, or the assembly of fibrous material and mold can be sealed and liquid resin drawn through the fibrous material by application of vacuum. Alternatively, the fibrous material can be pre-impregnated with a resin (again, a mixture of the resin per se and a suitable hardener) that is activated by an increase in temperature. These materials, which are commonly referred to as “prepregs”, or as comprising “B staged” resin, may be activated at room temperature, which requires that they be refrigerated prior to use, or may be inactive at room temperature and activated by heating after disposition in the proper configuration.
In each of these techniques the resin is cured after application to form a solid. As above, in some cases the curing takes place by application of heat, commonly by disposing the fibrous materials with resin preapplied in an oven or “autoclave”; in other cases the resin comprises a mixture of a resin component and a hardener (also known as a cross linker), mixed just prior to application, so that curing takes place by a cross linking chemical reaction between the two.
The strength, the stiffness, and durability of the structures thus formed is due primarily to the strength, that is, the modulus of elongation of the fibrous material, and requires that the matrix of cured resin maintain the fibers in their proper alignment after curing; the resin itself is typically somewhat flexible and of lesser strength.
More specifically, the load carrying capacity (the strength) of composites is a function of the tensile strength and the alignment of the fibers used. The stiffness is a function of the Young's modulus (sometimes called tensile modulus or elastic modulus) of the fibers. In general, the durability of a composite in a normal environment depends on how well the reinforcing fibers and the matrix are bonded. If the composite is to be used in a corrosive or abrasive environment, the material of the matrix plays a major role in durability. While the load carrying capacity and the stiffness of the structures thus formed is due primarily to the strength and modulus of elongation of the fibrous material, the durability of the composite component primarily depends on the compatibility between the fibrous material and the resin used. Thus it is essential that the matrix of cured resin maintain the fibers in their proper alignment after curing, which in turn requires good interfacial bonding between the resin and fiber matrix.
During the curing process the resin material undergoes a chemical reaction, whereby chains of resin molecules are bonded to one another to form a solid, and are also chemically bonded to the fibrous materials, so that the fibrous materials are locked in place by a matrix of the cured resin. The fibrous and resin materials must be chosen for compatibility, so as to obtain the maximum interfacial bonding between the matrix material and the fibrous material. As above, in such structures, the load-carrying capability of the end product is optimized by the strong fibers being properly aligned to take the stress, while the lower-strength matrix material secures the fibers in the proper position and assists in distributing the load between the fibers.
Commonly, the fibers are provided as cloth, typically in a “plain weave” construction, where yarns made up of numerous fibers are interwoven in “warp” and “weft” directions. As is well known, the plain weave method of weaving can be performed at high speed, reducing costs, and a selvage may be provided on the edges of the fabric, keeping the fabric stable. However, when the fabric is later cut to shape, the fabric can become unraveled easily as the yarns are not locked in position, as they are in a knitting process, for example. In order to simplify handling of the fibrous materials, and to ensure that, for example, the yarns making up a fabric maintain their proper relative disposition and alignment while being cut, and have maximum dimensional stability so that the yarn will not unravel from the fabric structure while being draped into position and the like, it is known to provide an adhesive on the fabric as a yarn locking mechanism. The adhesive provides dimensional stability to the fabric and prevents the fabric from becoming unraveled or buckling, and prevents the yarns from becoming misaligned or sagging during subsequent cutting and handling steps.
More specifically, some of the yarns to be woven into the fabric can be coated with a “hot melt” adhesive and heated while being woven, so as to form a grid providing dimensional stability to the fabric. The woven fabric containing parallel hot melt-coated yarns separated by, for example, 0.05-1″, allows the user to slit the fabric without concerns of fabric unravelling or loss of dimensional stability. This is a common practice in carbon yarn weaving where both edges of the fabric is locked in by two parallel hot melt-coated yarns on each side, separated by about 1″. Once the edges are slit, the remaining adhesive coated yarn on each side stabilizes the fabric.
In some circumstances, where different layers of fabric contact one another in order to build up the thickness of the structure to a desired degree, the adhesive may be employed by application of heat and pressure during the assembly process to secure the adjoining layers to one another, allowing the structure to be built-up as desired without having the various layers of fabric slip out of position. The built-up structure is then infused with a hardening resin (as above, typically a mixture of the resin per se and a hardener) and the resin allowed to cure, so as to form the final composite structure. See generally U.S. Pat. No. 5,212,010 to Curzio et al and U.S. Pat. No. 4,906,506 to Nishimura et al.
An adhesive commonly used for this purpose is ethylene-vinyl acetate (EVA), which is an inexpensive “hot melt” adhesive. Other hot melt adhesives, such as polyolefins, polyamides, and polyesters are suitable and are to be understood to be within the scope of this disclosure although only EVA is mentioned specifically, for brevity.
As noted, EVA is commonly applied to some fraction of the yarns woven into the fabric, typically by passing yarn through a bath of EVA and removing the excess EVA by passing the yarn through a sizing die. This results in a yarn completely encapsulated by EVA. The EVA-coated yarn can then be woven into the fabric together with uncoated yarns. The adhesive-coated yarns can be provided in both “machine” (i.e., warp) and “cross machine” (i.e., weft) directions. During weaving, the hot melt-coated yarns are passed through a heated nip roller, so that the adhesive yarns in machine and cross machine directions adhere to each other and to the other yarns of the fabric, forming a grid stabilizing the fabric during subsequent cutting and handling steps.
Furthermore, if the fabric is then disposed in a mold or the like, heat and pressure can be applied to the EVA-coated yarns, e.g., by a heated roller tool, so that the EVA-coated strands in one layer of fabric adhere to the yarns of the adjoining layer(s), securing the fabric layers in their desired position before the resin is applied.
Similar techniques may be used to stabilize and assemble tapes of unidirectional fibers.
However, a problem arising from the practice of using EVA-encapsulated yarns as part of a fabric or tape to be employed in forming a composite structure is that the resins commonly used do not form a chemical bond to the EVA. Accordingly, the strength of the composite structure is compromised by this practice. More specifically, when stress is later applied to the composite structure, the weakened points in the structure at which the EVA is present form “stress raisers”—that is, points at which a crack can be initiated.
Still more specifically, as is well known, any sudden discontinuity in the properties of a stressed member will cause stresses to be concentrated at that point. The EVA-coated yarns used in the prior art practice form such discontinuities in the part, and cracks frequently start at their locations. Moreover, a crack thus started can propagate along the length of the weakened line in the structure formed by the presence of an EVA-coated yarn, possibly leading to catastrophic failure.
It would be possible to employ an adhesive other than EVA, to which the resin would bond, or to provide additives to the EVA that would allow such bonding, but both of these alternatives add significantly to the cost.
It is therefore an object of the invention to provide a method for fabrication of composite structures that avoids this problem while preserving the several advantages of employment of the EVA adhesive mentioned above, that is, primarily to prevent unraveling of a woven fabric during cutting and handling, and secondarily to hold the components of a fabric or tape to one another and to other sections of fabric or tape while being placed into the desired configuration.